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Sunken Roofs

     Sunken floating roofs are a FLBSS {Flammable Liquid Bulk Storage Specialist} night-mare. Unless there is an explosion from a lightening strike, they have been rare. However, when they happen the FLBSS must be ready to act and advise. There are two general types of floating roofs for bulk storage tanks. Flat sealed roofs and sealed pontoon style roofs. Since the pontoon roof has an inner and outer roof panel, the ID {inside diameter} can contain more buoyant material that affords lighter overall construction. Both have excellent records with bulk flammables and have become the choice for retro-fits and new construction. Since both have stabilizing cable contacts inside the tank which prevents turning as they rise and fall with the product level, twisting and dipping an edge under the surface is eliminated. During an emergency, a damaged portion may collapse and sink into the tank. Many times a partial “sink-age” creates more challenges for fire suppression than a total “sink-age”. Hazard knowledge, safety procedures, and tactical guidelines can minimize these challenges.

     If there has not been a total failure of the roof, the majority of fires begin in the seal to tank wall contact edge. Here, a pliable seal makes contact with the tank wall and slides up and down the tank wall as the volume is increased or decreased. Occasionally, seals will become worn or fractured allowing vapors to escape and ignite. Another hazard involving the unintentional sinking of these roofs involves rain drainage. Since floating roofs are flat, they have interior tank drain pipes mounted to release rainfall accumulation. To avoid “sink-age” these must be opened during inclement weather conditions. If they are not, the roof area can be covered with water and cause the roof to become off-balanced. This increases the likelihood of a fully sunken roof. During fire situations the rain drains must be open and operating BEFORE water or Foam application is started. Your efforts of suppression can create the same imbalance as rainfall except much quicker.

     To prevent a full sinking of a floating roof the tank should have, and the tank farm personnel should; 1] Regularly inspect tank seals for integrity, 2] Insure cathodic protection for the entire system of the tank, 3] Insure seal area Foam chambers are in operating order, 4] Check flow rates of rain run-off lines and be sure these are unobstructed, and 5] Inspect roof integrity annually, and after every thunderstorm that has generated lightening close by.

     During a partial “sink-age” fire may still burn underneath hidden under sections of the roof that were damaged. When this happens, total extinguishment can be accomplished by allowing a burn off in conjunction with pump transfer of remaining product to another tank, burn pit, or containers. Without the ability to direct Foam resources to cover the entire surface, this becomes the resulting option for attack. During a total “sink-age” some challenges that occur are increased product levels, a

Pre-heated metal roof making contact with the bottom water layer, and obstruction of pump transfer lines by damaged portions of the sunken roof.

     If any or all of these occur, the resulting fire attack should include these considerations added to your action plan. 1] If the roof has sunk before your arrival, how long had the fire been burning? 2] Did the roof metal absorb enough heat to cause a boil-over concern? 3] If not, how full is the tank now? 4] Will the volume displacement from the sunken roof affect the volume of Foam application? 5] Is the pump product transfer line obstructed by roof fragments, or can the product be transferred?          

     The answers to these questions will allow you to establish a plan of action for fire attack. 1] If the fire had been burning for an extended time, appreciate that the potential exists for radiated heat from the roof material to “super-heat” the water layer at the tank bottom. 2] If this occurs, the water may boil in the near future, creating a boil-over situation. If this is a possibility, unmanned monitors and troop withdrawal may be the appropriate choice. 3] Does the tank have enough room in it to start a large scale Foam attack without causing an overflow? 4] If it does not, transfer the product to create the volume needed for this task. 5] If the pump transfer lines are blocked by roof sections, product transfer and Foam application may not be an option. At this point, fall-back, protect exposures, while allowing a “burn-out” may be the only option available. Unmanned cooling monitors should remain in operation through-out the burn to cool the exterior tank walls, to maintain the tanks integrity. However, if the line is operating, immediately transfer the product to create the needed volume, to begin your Foam attack.

                               Haz Mat Mike 

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